Installations for making X-ray photographs



May 10, 1960 G. D. A. HOEKSTRA 2,

INSTALLATIONS FOR MAKING X-RAY PHOTOGRAPHS Filed Oct. 30, 1957 A K V 200#4 United States Patent INSTALLATIONS FOR MAKING X-RAY PHOTOGRAPHSGerrit D. A. Hoekstra, Delft, Netherlands, assignor to N.V.Nederlandsche Rontgen-Apparatenfabriek, Delft, Netherlands ApplicationOctober 30, 1957, Serial No. 693,285 Claims priority, applicationNetherlands October 31, 1956 9 Claims. (Cl. 250-103) The inventionrelates to an installation for making X-ray photographs, in particularfor purposes of medical examination.

When making medical X-ray photographs or radiographs in the usual way,the object to be exposed is placed in front of a fluorescent screen andthe image appearing on this screen is photographed by means of a cameraarranged to the rear of the screen. In order to obtain an exposure ofhigh quality, it is essential that the photographic film has therequired density and that the image is sharply defined and has the rightdegree of contrast. This quality of the image is greatly influenced bythe quality, i.e. the hardness of the X-rays and the quantity thereof.The quality of the X-rays depends on the voltage of the tube (kv.) andthe quantity is determined by the product of the current intensity ofthe tube and the time of exposure (ma-see).

In the conventional method for making X-ray photographs, for each objecta distinct tube voltage is chosen,

in most cases as low a voltage as practical and with which it isexpected that a good picture can still be obtained. This is generallydone by measuring or weighing the patient to be radiographed anddetermining the desired tube voltage by means of a table. In connectionwith the value for the tube voltage so found (usually varying betweenca. 50 kv. and 125 kv.), the tube current is then so chosen thatin theshortest possible exposure time the required film density is obtained.

In the conventional X-ray installations the tube-voltage is, therefore,adjustable but has to be kept at a constant value during the exposure.This implies that an auxiliary voltage stabilizing apparatus is requiredto compensate possible fluctuations of the voltage of the networkfeeding'the installation. X-ray installations of the transportable kindwhich must, therefore, be connectable to different networks, also needan apparatus for adapting the installations to the resistance of thenetwork. Apart from the means for adjusting the tube voltage, a furthercomplicated circuit for protecting the X-ray tube against overloading isrequired. All these auxiliary devices and circuits make the installationcumbersome, complicated and expensive. i

. The invention has for its main object the provision of an X-rayinstallation for making X-ray photographs of the kind described which asregards operation and adjustment is considerably simpler and thereforequicker to handle as compared with the known installations.

Another important object of the invention is to provide an X-rayinstallation of the kind described of which the auxiliary apparatus canbe greatly reduced, thereby considerably reducing the weight, volume andmanufacturing cost of the installation.

X-ray installations of the type as discussed above normally comprise asource of high-voltage, such as a highvoltage transformer or otherhigh-voltage generator means to be fed by the network, whichhigh-voltage source. is connected across the anode and hot incandescent2,936,375 a [C6 Patented May 10, 1960 cathode of an X-ray tube at thestart of an exposure by the operation of an exposure switch or the likeand is again disconnected after a selected time of exposure by means ofa time switch means which may be of the type that has to be pre-setaccording to the desired exposure time or which may be of the knownautomatic photo-timer switch type to be discussed below. According tothe invention, the installation further comprises circuit elements whichimmediately after the switching on of the generator means by theactuation of the exposure switch cause the voltage across the tube torise quickly from a selected minimum value and the current through thetube to fall from a selected maximum value.

At the start of the exposure, the tube voltage is thus relatively low,for instance 50 kv., the voltage however rising quickly, in say 0.2sec., towards a maximum value of, for instance, -125 kv. Generally,however, this maximum value of the tube voltage will not be reachedbecause-before that the film will already have obtained the desired filmdensity so that the tube is disconnected by the time switch means at alower voltage. Children and thin persons are thus exposed andphotographed at a relatively low average tube voltage and acorrespondingly high average tube current, comparable to theconventional exposure methods, which provides a good picture quality.When exposing heavier persons, the required greater X-ray dose isobtained in that towards the end of the longer exposure time as a resultof the rising the requirements of the conventional radiographicalexposure techniques, whereas insofar as these requirements are not fullymet the additional dose of harder rays serves as a correcting factor.

By properly selecting and adjusting the relevant circuit elements, thetube-voltage-time curve and tube current-time curve during exposure canbe made to follow such a course that when at the end of the exposure therequired film density is reached, the picture at the same time has aright degree of contrast.

An especially advantageous result is obtained when, according to apreferred embodiment of the invention, a photo-timer switch arrangementis used for controlling the exposure time. Such a photo-timer switcharrangement, in itself well known in the art, comprises a photo,-

electric device which cumulatively measures the quantity with the typeof film used, it is thus possible to have the exposure finished atexactly the moment the right film density is reached. t

In this way a practically automatically functioning X-ray installationis obtained. The patient need no longer be weighed or measured in orderto determine the correct values of the tube voltage, current and exposure time and to adjust the installation accordingly,'

but without any adjusting operation the patient can immediately beplaced in front of the screen and the exposure switch be operatedwhereupon the photo-timer switch disconnects the X-ray apparatus afterthe correct 1 exposure time." This photo-timer switch arrangement in 3cooperation with the described novel characteristics of the'X-rayinstallation thus, as it were, automaticallyse lects the desired averagetube voltage for the object concerned.

'It follows from the foregoing that fluctuations of the network voltagearound the nominal value' thereof cannot aversely affect the quality ofthe resulting picture which means that a tube voltage stabilizingapparatus can be dispensed with. Only the voltage of the heating circuitof the X-ray tube must be stabilized which can, however, be done withrelatively simple means.

According to a preferred embodiment of the invention, the installationcomprises circuit elements which after the switching on of the X-raytube cause the tube current to fall sharply, while the high-voltagegenerator means has a high impedance which is so chosen that the tubecurrent fall effects the desired quick tube voltage rise. When, forinstance, the generator means comprises a high-voltage transformer, thelatter is given a high internal resistance so as to obtain a sharplydropping current-voltage characteristic of the transformer output.Because of this high impedance or high internal resistance of thetransformer, variations in the resistance of the network to which theinstallation is connected, will no longer have any real effect so thatthe otherwise necessary apparatus for adapting the installation to thenetwork resistance can be dispensed with.

The tube-current fall can be easily realized'by decreasing the voltageacross the heater of the X-ray tube in one step from a high to a lowlevel at the moment the exposure switch is operated. The tube cathodewill thereby quickly cool and the tube current 'fall correspondingly toa lower level of intensity.

As follows from the foregoing, the. high-voltage generator means ortransformer can during an exposure directly be connected to the network,i.e. without the interposition of any voltage regulating or stabilizingdevices. It is, furthermore, possible to adapt the highvoltagetransformer to the characteristics of the X-ray tube in such a way thatthe focus of the latter cannot be overloaded but that, on the otherhand, during exposure themaximum admissible load is approached. For themaximum power of the X-ray tube can be expressed by the product of thetube voltage and the accompanying tube current and decreases with time.By a proper selection of the current-voltage characteristic of thehigh-voltagegenerator means and the time constant of the relevantelements of the installation it is possible to.insure that the maximumadrnissable value of this product can at no time be-surpassed. In thatcase, the apparatus for protecting the tube against overloading canlikewise be dispensed with. As a result, the X-ray installationaccording to the invention can be made small, light andeasilytransportable, while, furthermore, because of'its relatively simpleconstruction, the possibility of a fault or break-down is greatlyreduced.

These and other novel features of the invention will appear more fullyfrom the following description and claims in conjunction with thedrawing which shows an embodiment of the invention.

In the drawing:

Fig. l is a schematic diagram of connections of an X-ray installationprovided in accordance with the in? vention; v

Fig. 2 represents the current-voltage characteristic of the high-voltagetransformer of the diagram of Fig. 1; and

Fig. 3 illustrates the tube-voltage curve and'tube-current curve asplotted against exposure time.

The X-ray installation diagrammatically shown in Fig. 1.has' a hightension transformer 1 with a primary winding 2 and a secondary winding 3provided with a grounded center tap. The winding 3 has a highresistancegiving the transformer on its secondary side a current-voltagecharacteristic as shown somewhat idealized in Fig. 2, i.e, at adecrease'of the secondary transformer currentfrom about 200 ma. to 40ma., the secondary transformer voltage rises from about 50 kv. to kv.The trans former winding 3 feeds a biphase full-wave rectifierarrangement 4, the DC. terminals of which are connected to the cathode 5and anode 6, respectively, of an X-ray tube 7.

The primary transformer winding 2 is fed from a network 8 through a mainswitch .9 and leads 10 comprising a pair of contacts 11 of a relayswitch 12 having an actua= tion coil 13.

V The heater of cathode 5 of the tube 7 is fed by a heating currenttransformer 14 which on its primary side is connected by leads 15 totheleads 10-through a variable resistance 16 for the adjustment of theheating current:

on the one hand and through a second variable resistance 17 on the otherhand. The latter resistance 17 can be bridged by a contact 18 of a relayswitch 19 having a coil 20.

Coil 13 of relay switch 12 is connected to the. leads. 10 through aseries connection of a contact 21 of a relay,

switch 22 having a relay coil 23 and contacts 24, 25 of:a doublemanually operated exposure switch 26. The mov-- able contact 24: of theswitch 26 further cooperates with a. contact strip 27 in such a way thatthis switch has an" intermediate or pre-exposure position between itsillustrated position and its exposure position in which preexposureposition contact 24 only touches contact strip 27 whereas in itsexposure position the movable switch contact 24 is connected both tocontacts 27 and 25; Switch 26 has a second contact arm carrying amovablecontact 28 which in the exposure position of the switchisconnected with a further fixed switch contact 29.

Coil 20 of relay 19 is connected to leads 10 through V the contacts 24,27 of the exposure switch 26, a lead 30,

' a contact 31 of the relay switch 12 and a contact 32 of a furtherrelay 33 on the one side and through a lead 34 on the other side. Thecoil 35 of this relay 33 is connected to leads 10 through lead 34, andthrough a further. movable contact 36 of relay switch 12 and a lead 37.Re. lay 33 has a holding contact 38 comprised in a lead 39. leading fromone end of the relay coil 33 to the contact strip 27 of the exposureswitch 26. V

The relay 22 is controlled by a photo-timer switch arrangement of wellknown type comprising a photo-electric cell 40, a condenser 41 connectedacross this cell to, be charged by the latter and a thyratron 42controlled by the voltage of the condenser 41- which is connectedacross, the gridand cathode thereof. The output circuit of the.thyratron 42 comprises thecoil 23 of relay 22 in serieswiththe contacts28, 29 of the exposure switch 26.- The photo-electric cell 40 is soarranged as to receive light radiated by a portion of the pictureappearing on the;

fluorescent screen S of the installation during the exposure. ofanobject placed in front of this screen which picture:. is at the sametime photographed by a camera arranged: together with the cell to therear of this screen. The, construction and relative arrangement of theelements .of. the radiographical outfit comprising an X-ray tube,fluores'cent screen, camera and photo-timer arrangement is,

well known in the art and is, therefore, not shown; and. described indetail.

The working of the installation represented in Fig. 1. is as follows.

When the main switch 9 is closed, the X-ray tube-7- receives a reducedheating current through leads 15 and transformer 14, the valve of thiscurrent being determined by the setting of the variable resistances 16and 17. The

whereby a circuit is closed from the upper one of leads... 10, throughcontacts 24 and 27 of the exposure switch,

lead 30, contact 31 of relay switch 13, contact 32 of relay 38,- coil20of relay-19 and back'through lead 34"to the" otherone leads 10.RelayJ-Q, thus excited-closes contact flishort circuiting the resistance17, whereby the'voltage at the heating current transformer 14 is raisedto its full working value as adjusted by the variable resistance 1,6;and the-tube7 is fully heatedup: 1

Thereupon the switch 26 is moved to its exposure position, its contacts24, 25 thus closing the circuit through cont'act21 of relay 22 and thecoil 13 of relay switch 12 immediately closes-its contacts 11 and 36 andopens contact 31.- By the closing of contacts 11, the X-ray tube 7 isput under voltage. At this moment, the tube current as determined by theinitial full heating current voltageis, for instance, 200 ma. and thetube voltage 50 kv.

The opening of relay switch contact 31 at the same time interrupts thecircuit of relay coil 20 whereby relay contact 18 is opened withoutdelay and the resistance 17 is again connected in the heating currentcircuit. The voltage across the transformer 14 thus drops to a lowerlevel, causing a gradual cooling of the tube cathode, whereby the tubecurrent decreases within about 0.2 see. from its top value of 200 ma. toa lower level of about 40 ma. according to curve 43 of Fig. 3. Becauseof the high internal resistance of the transformer 1, the tube voltagewill correspondingly rise from 50 kv. to 100 kv. according to curve 44of Fig. 2.

Before these latter values of the tube voltage and tube current arereached, however, the X-ray tube will generally be disconnected by thephoto-timer switch arrange ment 22, 4042. Part of the light radiated bythe fluorescent screen S during exposure is received by thephoto-electric cell 40, and the condenser 41 is thus charged by theresulting current flowing through the cell circuit, the speed ofcharging being dependent on the quantity of light received by the cell40. As soon as the voltage across the condenser 41 has reached aselected valve, the thyratron 42 tires, whereby relay coil 23 isexcited, opening its relay contact 21. The circuit of relay switch coil13 is thus interrupted and the high-voltage transformer 1 disconnected.By properly selecting and adjusting the elements of the photo-timerswitch circuits with respect to the properties of the film used forphotographing the image appearing on the fluorescent screen S thisdisconnection can be made to occur exactly at the moment the right filmdensity has been obtained.

The return of the several contacts of relay switch 12 to their initialposition at the end of the exposure time does not affect relay 33 sinceits coil 35 remains excited through its holding contact 38. Contact 32therefore remains open and the circuit of relay coil 20 interrupted,thereby preventing contact- 18 from closing and again short-circuitingresistance 17 the moment the transformer 1 is disconnected by switch 12.Relay coil 23 keeps drawing current through the fired thyratron 42 andholds its contact 21 open, thus preventing switch 12 from re-closing itscontacts 11. Only by moving the exposure switch 26 back to its initialneutral position can the circuits of relay coils 35 and 23 beinterrupted, whereupon the next exposure can be made in the same manneras described.

Obviously, it is possible to replace the photo-timer switch arrangementby a normal clockwork time switch in which case this switch has to beadjusted to the proper exposure time prior to the exposure.

The form of the tube voltage-time and tube currenttime curves 43 and 44of Fig. 3 can be so chosen that no matter after. what exposure time(i.a. at which point of the curves), the right film density is reachedand the X- ray tube disconnected, the photographic picture also has aright degree of contrast; furthermore, the form of the curves can be sochosen as to prevent the possibility of overloading the X-ray tube nomatter how long the exposure, since the momentary load is a function ofthe value of the tube voltage and the intensity of the tube currentoccurring at each given moment. The form of the curves 43 and 44 isdetermined by the time constants and characteristics of the variouscircuit elements involved. More particularly, theirformdepends on thetemperature-time curve of the cathode of the X-ray tube and on theproper selection and adaptation of the current-voltage characteristic ofthe transformer 1 with respect to thistemperature-time curve. After thetube and the transformer have been selected, however, some adjustment ofthe curves 43 and 44 remains possible within a small range by variationof the resistance 17 and, to a lesser extent, of the resistance 16.

The described installation of Figure 1 needs no stabilizing circuit forthe tube voltage. A device for stabilizingthe voltage of the heatercurrent circuit may possibly be required but is not shown; such a deviceis relatively simple and can be of any appropriate known kind.

It will be obvious to those skilled in the art that variousmodifications of the embodiment of the invention as shown and describedcan be resorted to within the scope of the following claims.

What is claimed is:

l. A method of controlling an X-ray tube for making an X-ray exposure ona photographic film comprising, during exposure of said film to theX-rays, continuously increasing the anode voltage of the tube from aselected minimum value and simultaneously continuously decreasing thecurrent flow from a selected maximum value and disconnecting the tubewhen the desired exposure of the film, for the subject being X-rayed, isattained.

' 2. An X-raying method for making an exposure of a photographic filmcomprising applying an anode voltage to an X-ray tube for the generationof X-rays to obtain an exposure on said film of the subject to beX-rayed, applying to the cathode of the tube during exposure of the filma heater voltage and decreasing the heater voltage during the filmexposure so as to decrease the cathode temperature whereby current flowthrough the tube de creases and the anode voltage thereof increases in acontinuous manner thereby similarly increasing the intensity of theX-rays, and disconnecting the X-ray tube at the end of the exposure.

3. A method as claimed in claim 2 comprising measuring the exposurecaused by the X-rays as affected by the subject being X-rayed anddisconnecting the tube when a predetermined exposure is obtained.

4. Apparatus 'for X-ray exposures of a photographic film comprising anX-ray tube, means coupled to said tube for applying operating voltagethereto, means coupled to said tube for continuously decreasing thecurrent passing therethrough during exposure, means coupled to said tubefor causing the anode voltage of the tube to increase continuously onsaid decrease of said tube current, and means coupled to said means forapplying operating voltages to disconnect said tube at the end of anexposure.

5. Apparatus as claimed in claim 4 in which said means for disconnectingthe tube comprises means for sensing said exposure so as to cause thetube to be automatically disconnected when thedesired exposure of thesubject being X-rayed is obtained.

6. X-ray apparatus for exposure of a photographic film comprising anX-ray tube including an. anode, cathode, means to heat said cathode, avoltage source coupled to the anode and a control coupled to said meansfor lowering the heating of the cathode during the film exposure andthus decreasing the current flowing through the tube, and means coupledto said tube for increasing the anode volt-age when said tube currentdecreases.

7. Apparatus as claimed in claim 6 wherein the first said meanscomprises a heating circuit, a resistor in the heating circuit andshort-circuiting means to by-pass said resistor, said control comprisinga switch device coupled to said short-circuiting means to control theby-passing of said resistor.

8. X-ray apparatus comprising an X-ray tube including an anodeand athermal responsive electron emission electrode a voltage source;aresistor coupling 'said'source' to said emission electrode; a higi'iimpedance devi'ce for coupling said source to said anode; means forlay-passing said resistor at 'least in part, a-switch' for coupling thesource to the anode via's-aid high impedance device a'nd actuating saidmeanswhereby the current through said tubeis decreased and the voltageat said anode increased during exposure of a photographic film, meansfor registering the exposure of a subject, means operatively disposedwith respect to the second said means for sensing said exposure andbeing coupled to said source for disconnecting the tube from said sourcewhen an exposure of predetermined magnitude has been obtained.

2054,4 93 2,319,322 7 Hefel May; 1-18, 1943;- 2,379,1 25 Weisglass June26, 1945. 2,617,047 Kuntke Nov; 4, 1952; 2,667,587 K'untke. et a1, Jan26,=19,-5 2,681,417

Ball June 15, 1955;;

